This invention relates to electrostatography, and more particularly to an apparatus for enhancing the transferability of a developed latent image from a photoconductive surface to a receiving member, and for suppressing the transferability of background particles to the receiving member.
In the known practice of xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic image. In the usual method of carrying out the process, the xerographic surface is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The discharged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.
The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the largest amoung of material is deposited; and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image on the copy being reproduced. The powder is subsequently transferred to a sheet of paper of other surface and suitably affixed to thereby form a permanent print.
The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred herein to as "toner" and a coarse granular material called "carrier". The carrier is coated with a material removed in the tribo-electric series from the toner so that a charge is generated between the powder and the granular carrier upon mutual interaction. Such charge causes the powder to adhere to the carrier. The carrier, besides providing a charge to the toner, permits mechanical control so that the toner can readily be brought into contact with the exposed xerographic surface for the development of the surface. The powder particles are attracted to the electrostatic image from the granular material to produce a visible powdered image on the xerographic surface.
A conventional technique for transferring toner from a photosensitive surface to a copy sheet is to move the sheet into synchronous contact with the photoconductive surface while concurrently applying a charge opposite in polarity to the toner to the side of the paper remote from the photoconductive surface. The toner image is thereby attracted from the surface of the photoconductor to the copy sheet. A puff of air may then be employed to separate the image bearing copy paper from the photoconductive surface. The toner image is then fused for the production of the final xerographic copy. This procedure is described in more detail in U.S. Pat. No. 3,062,536.
If negatively charged toner is employed in the system, the transfer corotron is biased positively to deposit a uniform positive charge across one side of the copy paper. With such an arrangement, the negatively charged toner in the image areas of the developed image form areas of high negative charge and are strongly attracted to the copy paper. Background areas of the developed image have only a small amount of toner carried thereby and are only weakly attracted to the copy paper. Even though toner in the background areas is only weakly attracted, some nevertheless is transferred to the copy paper, and this detracts from copy quality.
A method of overcoming this transfer of toner in background areas is disclosed in U.S. Patent application Ser. No. 440,409, assigned to the assignee of this application. In that method, transfer of background toner is inhibited by exposing the photoconductive surface after development, but prior to transfer, to a corona discharge device which neutralizes the negative charge on the toner in background areas while increasing the negative charge on toner in image areas. This method employs an A.C. corotron biased to a positive or negative D.C. potential. The effectiveness of this selective pre-transfer process, is dependent in large measure on the output characteristics of the pre-transfer device, and this in turn must be selected by considering the relative potential of the developed photoconductive surface in the image as opposed to the background areas. That is, if the original to be copied has good contrast, the average density of the toner deposited in image or information bearing areas (and consequently the plate potential) will be high and the output of the pre-transfer corotron may be adjusted to suppress the transfer of all toner densities less than this relatively high value. However, if the original has poor contrast characteristics, the average density of information areas may be significantly lower, such that if the pre-transfer corotrons noted previously is employed, transfer suppression of this toner may take place resulting in a loss of information in the copy.
Thus, when designing a selective pre-transfer corotron for use in a given machine, a compromise must, of necessity, be made so that given the "normal" or "average" original, transferability of image toner is enhanced, while that of background toner is reduced. In such a case, when an original having substantially more or less contrast is copied, less than optimum results are obtained from the selective pre-transfer step.